Frequently it is necessary to transmit a voice communication from a mobile unit while using an audible warning device such as an electronic siren. When the siren is actuated there will be an acoustical path to the mobile microphone. Thus when the mobile operator transmits a voice message, the voice signal will be combined with the background siren signal. This combination will impair the intelligibility of the voice signal perhaps even masking out important emergency information.
The siren signal cannot be merely substrated out due to the unpredicability of the magnitude and phase of the background siren tone at the mobile microphone. Thus the siren signal must be notched out of the composite siren and voice signal.
An example of a conventional method of notching out an undesired frequency from a multi-frequency composite signal is described in U.S. Pat. No. 4,453,137 to Rittenbach. In this patent a signal processor circuit is described which includes a plurality of cascaded stages which are used to notch out a certain frequency from a multi-frequency composite signal. In Rittenbach a conventional frequency locked loop is utilized. Each of the stages contained the composite signal which is contained inside of the frequency lock loop.
Applying the system in Rittenbach to a situation as heretofore described would result in the PA speaker of the electronic siren to mobile microphone audio path being part of the frequency locked loop. Since the siren to microphone feedback loop is highly unpredictable and is variable, this would cause great difficulties in acquiring and maintaining a lock on the siren signal. Although Rittenbach is able to lock on a particular frequency of the composite multi-frequency signal, it would not be able to do so in a situation wherein one of the composite signals was an audio signal from an electronic siren being input at a microphone together with a voice signal. Thus, in this situation, the notch filter as taught by Rittenbach would not improve the intelligibility of the radio operator's voice transmission.
In emergency situations every syllable of the voice transmission could be critical. Thus there exists a need to provide a tracking audio filter for notching out a background electronic siren from the voice transmission to improve intelligibility. Such a filter would be widely received in the industry.